Method of plasma treating a polymer film to change its properties

ABSTRACT

A method of treating a polymer film to alter its physical properties, comprising exposing the film to an ionized plasma in a vacuum environment with the ionizing gas producing one of surface etching, polymer cross-linking and coating of the film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a method of treating a polymer filmwith a low temperature, e.g., 50°-120° Celsius, plasma to improve itsphysical properties, such as tensile strength.

2. Discussion of the Prior Art

Plastics films have attained a considerable degree of commercial successand are used in a wide variety of products. One such use is in theformation of bags where the film is subjected to considerable forceswhen in use. One film strength characteristic which is important,particularly in bag manufacture, is tensile strength. However, it hasbeen found that tensile strength of a film can be adversely affected bythe presence of low molecular weight polymer species which migrate tothe film surface during melt crystallization of the polymer forming thefilm. These short chain molecules have fewer polymer entanglements thanthe underlying bulk material and therefore film failure is more easilyinitiated at a lower stress level than would be the case with theunderlying film bulk material. A film failure initiated in the surfacelayer easily propagates into the underlying bulk material. In addition,the extrusion of molten polymer into a high flow of cooling air, such asoccurs in blown film extrusion, causes surface embrittlement throughoxidation. A brittle polymer surface coating on a more ductile substratealso reduces physical properties of a film, e.g., tensile strength, byincreasing the ease of fracture initiation.

Several techniques, such as molecular orientation, uniaxial and biaxialstretching, etc. have been used to increase physical properties of afilm. In general, however, these strength increasing techniques do notaddress the inherent weakness problems caused by a surface layer of afilm which is more easily fractured than an underlying bulk materiallayer.

SUMMARY OF THE INVENTION

One general object of the invention is the provision of a method oftreating a polymer film with a plasma environment to alter its physicalproperties.

Another, more specific, object of the present invention is the provisionof a method of treating a polymer film with a plasma environment toalter its physical properties by removing weak or brittle surface layersthereof and increasing its inherent tensile strength.

These objects are obtained in a method which comprises the steps oftransporting a polymer film into a vacuum treatment chamber filled witha low concentration treatment gas, e.g., 1×10⁻⁵ to 4×10⁻⁵ gram moles perliter, at an absolute pressure of 0.2-0.7 Torr, ionizing the treatmentgas to create a reactive medium to which the film is exposed, exposingthe film to the reactive medium for a predetermined period of time andthereafter removing the film from the treatment chamber. The method canbe carried out on a continuous or discontinuous basis, and sequentialtreatments with different treatment gases can be done. The plasma gascan be an inorganic compound, e.g., O₂, N₂, AR, or organic compounds,e.g., silanes, saturated and unsaturated hydrocarbons, and aromatics.Modification of the film properties occurs by etching, cross-linking orcoating of the film surface, depending on the treatment gas employed. Toimprove the tensile properties of the film, an O₂ treatment gas, whichetches away a film surface layer, is preferred.

The method of the invention provides distinct property improvements withlittle or no material consumption. In addition, with etching, filmdowngauging can be achieved, while improving physical properties such astensile strength.

The method of the invention and its advantages and features will be moreclearly understood from the following detailed description, which isprovided in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an apparatus used to carry out themethod of the invention;

FIG. 2 is a front cross-sectional view of a treatment chamber used inthe method of the invention; and

FIG. 3 is a side view of the treatment chamber.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an apparatus which is used to practice the method ofthe invention. A vacuum treatment chamber 11 contains electrodes 12which are spaced by a fixed distance, e.g., 18". A film 14 to be treatedis disposed between the electrodes. The treatment chamber is connectedto a vacuum pump 23 through a vacuum trap 21. Vacuum trap 21 is a wellknown device which functions to collect condensible gases as a liquid,removing them from the treatment chamber 11 exit stream. Vacuum pump 23is used to evacuate the treatment chamber. One of the electrodes 12 isgrounded, while the other is connected to one output of a powertransformer 19, the other output of which is also grounded.

Before film treatment the vacuum chamber 11 is purged one or more timeswith a treatment gas. The treatment gas originates from one of aplurality of selectable sources, for example, an inert gas source 13,and an unsaturated monomer gas source 15 by means of respectiveselection valves 20,22 and flow meters. Another source of a monomertreatment gas 17, which is also selectable, is also provided andconnected to the treatment chamber through a precision metering valve25. Source 17 provides a monomer gas vapor from a liquid material, e.g.,from a flask containing 100-150 ml of liquid, with the vapor pressurethereof being controlled by the temperature of a temperature bath 18which surrounds the flask. Opening one of valves 20, 22 or 25 controlsthe application of a selected treatment gas to the vacuum chamber 11.Alternatively, a selected gas mixture formed by gases from two or moreof the sources, 13, 15 and 17 can be obtained by appropriate operationof valves 20, 22 and 25.

The treatment chamber 11 is illustrated in greater detail in FIGS. 2 and3. A film 14 to be treated is unwound from a supply roll 31, passes overa guide roll 33 and is wound on a take-up roll 35. The film passesbetween the electrodes 12 in passing from the supply 31 to take-up rolls35. The film can be driven through chamber 11 either continuously orintermittently by a motor 37 which is connected to the supply andtake-up rolls 31,35 by a driving assembly 38.

For film surface treatment, the treatment chamber 11 is initially purged2 or 3 times, with the selected treatment gas by selectively opening oneof the valves 20,22,25 and thereafter evacuating the chamber. Thetreatment gas then fills the chamber under 0.2-0.7 Torr absolutepressure. The treatment gas within the chamber is at a relatively lowconcentration in the range of 1×10⁻⁵ to 4×10⁻⁵ gram moles per liter. Thepower supplied by transformer 19 is then increased at the given gaspressure to cause an electrical discharge. Typically, the required powerwill range from 10 to several hundred watts. The resulting ionized gasbecomes the reactive medium. By varying the drive speed of film 14through the treatment chamber 11, the exposure time of the film to theplasma can be varied from a few seconds to several minutes, thusproviding a convenient means for varying the treatment level. Thermaldegredation of the polymeric film is prevented because the plasmareaction is carried out at low temperature, e.g., about 50°-120°Celsius, due to the reduced gas pressure (0.2 to 0.7 Torr) and the useof a low concentration (1×10⁻⁵ to 4×10⁻⁵ gram moles per liter) ofionized gas.

The plasma gas can be inorganic or organic compounds. As examples ofinorganic gas compounds, oxygen, nitrogen, helium, neon and argon can beused. Exemplary organic compounds include silanes, saturated andunsaturated hydrocarbons and aromatics.

The ionized gas causes modifications to occur at the film 14 surface byetching, cross-linking, or film coating, depending on the treatment gaswhich is used. For example, to improve the tensile properties of thefilm, an oxygen gas atmosphere is preferred, which results in an etchingaway of outer layers of the film. Typically, etching occurs to a depthof less than 1 micron. With O₂ etching, low molecular weight polymerspecies which have migrated to the film surface during meltcrystallization are removed, thereby increasing the stress levelrequired to initiate film fracture. Brittle layers caused by surfaceoxidation, which occur during blown film extrusion, are also removed. Ifan ethylene gas is used as the treatment gas, surface polymerization ofthe film occurs, with the film then being coated with a polyethylenelayer. Thus, it is possible to first use an oxygen gas plasma treatmentto remove brittle surface layers from a film and then use an ethyleneplasma treatment to produce a new polyethylene surface layer.

Various plastics films can be treated using the method of the invention,exemplary films being linear low density polyethylene, low densitypolyethylene, high density polyethylene, high molecular weight highdensity polyethylene, polypropylene, polystyrene and others. It is alsopossible to plasma treat non-plastic films and other articles. Forexample, an ethylene treatment gas can be used to form a polyethylenecoating on paper and wood products.

While oxygen gas (O₂) has been found to provide a surface etchingphenomena, the use of argon gas (Ar) has been found to inducecross-linking of the surface polymer. The cross-linking phenomenon hasbeen observed with various other inert gases, such as helium and neon.Organic monomers, when used as the treatment gas, provide surfacecoatings on the polymer film. Coatings may be applied in the mannerdescribed above by first activating the film surface by the use of anorganic or inorganic plasma, e.g., oxygen gas, after which the surfaceis contacted with the reactive monomer gas as the treatment gas. It isalso possible to first employ an ionized gas (plasma) treatment and thenexpose the film to a non-ionized treatment gas.

Table I illustrates experiments performed on seven polymer film samplesusing argon and oxygen as the first treatment gases and, in someinstances, with hexane and ethylene used as subsequent treatment gasesfor film coating, while Table II illustrates the changes in physicalproperties which were observed. In Table I, coating treatment with anon-ionized gas is illustrated by the symbol (-) under the voltage,current and power columns.

                                      TABLE I                                     __________________________________________________________________________    Experimental Conditions During Plasma                                         (Conditions During Coating)                                                                   Gas                                                                           Pressure                                                                           Voltage                                                                            Current                                                                             Power                                                                             Exposure Time                             Example                                                                            Film.sup.1                                                                         Plasma Gas                                                                          MilliTorr                                                                          Volts                                                                              MilliAmps                                                                           Watts                                                                             Seconds                                   __________________________________________________________________________    1    LLDPE                                                                              Argon 1000 300  100   30  20                                                  (Hexane)                                                                            (Conditions Not Recorded)                                                                         20                                        2    LLDPE                                                                              Argon 600  500  120   60  15                                                  (Ethylene)                                                                          (6200)                                                                             (-)  (-)   (-) (15)                                      3    LLDPE                                                                              Argon 610  440  120   53  72                                        4    LLDPE                                                                              Argon 650  430  122   52   5                                        5    LLDPE                                                                              Oxygen                                                                              590  700   70   49  72                                        6    LLDPE                                                                              Oxygen                                                                              600  600   80   48  72                                                  (Ethylene)                                                                          (7000)                                                                             (-)  (-)   (-) (72)                                      7    HDPE Argon 620  500  142   71  26                                        __________________________________________________________________________     .sup.1 Resins Used  LLDPE  Dow 2045/HDPE  DuPont 7810                    

                                      TABLE II                                    __________________________________________________________________________    Effect of Plasma Surface Treatments of Physical Properties of Thin Films      (Untreated Control)                                                                                                      Punc-                                                                         ture                                                                    Elmendorf                                                                           Resis-                                     Plasma                                                                             Coating                                                                            Yield                                                                             Ultimate                                                                           Elong                                                                             Toughness                                                                           Tear  tance                              Ex.                                                                              Film Gas  Gas  psi psi  %   Ft-Lb/In.sup.3                                                                      Gram  Lb                                 __________________________________________________________________________    1  LLDPE                                                                              Argon                                                                              Hexane                                                                             1965                                                                              4430 560 1200  129   --                                                   (1763)                                                                            (3529)                                                                             (514)                                                                             (962) (132)                                    2  LLDPE                                                                              Argon                                                                              Ethylene                                                                           1883                                                                              4491 585 --    --    --                                                   (1801)                                                                            (3772)                                                                             (540)                                              3  LLDPE                                                                              Argon                                                                              --   1770                                                                              3652 528 1013  128   --                                                   (1793)                                                                            (3529)                                                                             (514)                                                                             (962) (132)                                    4  LLDPE                                                                              Argon                                                                              --   1752                                                                              3718 537 1029  127   --                                                   (1793)                                                                            (3529)                                                                             (514)                                                                             (962) (132)                                    5  LLDPE                                                                              Oxygen                                                                             --   1742                                                                              4047 565 1165  132   --                                                   (1793)                                                                            (3529)                                                                             (514)                                                                             (962) (132)                                    6  LLDPE                                                                              Oxygen                                                                             Ethylene                                                                           1733                                                                              4426 641 1305  123                                                        (1793)                                                                            (3529)                                                                             (514)                                                                             (962) (132)                                    7  HDPE Argon                                                                              --   2967                                                                              6434 452 1475  14.1  1.62                                                 (3031)                                                                            (6666)                                                                             (459)                                                                             (1534)                                                                              (14.5)                                                                              (1.28)                             __________________________________________________________________________

As shown in the tables above, the cross-linking produced with an argongas treatment alone (examples 3, 4 and 7) produced minor changes in thetensile and tear properties of the films; however, the use of oxygenalong or with subsequent organic coating (examples 5,6) produced asignificant increase in tensile strength without changing elongation ortear properties. Sample 7 illustrates that although tensile strengthremains constant after argon treatment (cross-linking), punctureresistance increased significantly (30%).

While preferred embodiments of the method of the invention have beendescribed above, many modifications can be made thereto withoutdeparting from its spirit and scope. Accordingly, the invention is notlimited by the foregoing description, but is only limited by the scopeof the appended claims.

I claim:
 1. A method of strengthening a polymer film comprising thesteps of:placing said film in a vacuum chamber containing a pair ofelectrodes; filling said chamber with a treatment gas under a pressureof 0.2-0.7 Torr; applying a voltage to said electrodes to initiate adischarge in and ionization of said treatment gas; and exposing saidfilm to said ionized treatment gas for a predetermined period of timeand thereby etching away a surface layer of said film.
 2. A method as inclaim 1, wherein said treatment gas is an inorganic compound.
 3. Amethod as in claim 2, wherein said treatment gas is selected from thegroup consisting of oxygen, nitrogen, neon, helium and argon.
 4. Amethod as in claim 1, wherein said treatment gas is an organic compound.5. A method as in claim 4, wherein said treatment gas is selected fromthe group consisting of silanes, saturated and unsaturated hydrocarbonsand aromatics.
 6. A method as in claim 1, wherein said film is firstexposed to said ionized treatment gas and is thereafter exposed toanother ionized treatment gas.
 7. A method as in claim 1, wherein saidetching is to a depth of less than 1 micron.
 8. A method as in claim 1,wherein said film is selected from the group consisting of linear lowdensity polyethylene, low density polyethylene, high densitypolyethylene, high molecular weight high density polyethylene,polypropylene and polystyrene.
 9. A method as in claim 1, wherein saidtreatment gas is in a concentration of 1×10⁻⁵ to 4×10⁻⁵ gram moles perliter in said chamber.
 10. A method as in claim 1, wherein thetemperature of said ionized treatment gas is in a range of 50°-120°Celsius.
 11. A method as in claim 1, wherein said treatment gas in aninert gas.
 12. A method as in claim 5, wherein said treatment gas is anethylene gas.
 13. A method as in claim 1, further comprising the step ofremoving said treatment gas from said chamber, introducing a differenttreatment gas into said chamber and applying a voltage to saidelectrodes to initiate a discharge and an ionization of said differenttreatment gas.
 14. A method as in claim 13, wherein said treatment gasis oxygen and said different treatment gas is an ethylene gas.
 15. Amethod as in claim 1, further comprising the steps of removing saidtreatment gas from said chamber and introducing a different treatmentgas into said chamber which is not ionized.
 16. A method as in claim 15,wherein said treatment gas is oxygen and said different treatment gas isan ethylene gas.